<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    if (true) {
      const a = 10;
      console.log(a);	// 10
    }
    console.log(a);	// a is not defined
  </script>
</body>
</html>